A stilbene-based membrane spanning conjugated oligoelectrolyte 4,4'-bis(4'-N,N-bis(6"-(N,N,N-trimethyl ammonium) hexyl) amino)-styryl) stilbene tetraiodide (DSSN+) has been reported to be able to interact with bacterial cells and enhance their bioelectricity generation in bioelectrochemical devices, although the mechanism remains elusive. The goal of this study was to elucidate the impacts of DSSN+ on extracellular bioactivity and the underlying mechanism. Specifically, extracellular ferrihydrite reduction by Shewanella oneidensis was used to evaluate the influence of cell-DSSN+ interaction. Our results show that DSSN+ enhanced ferrihydrite reduction by S. oneidensis in a growth-dependent manner. The incorporation of DSSN+ into S. oneidensis cell membrane increased the extracellular concentration of redox shuttles, i.e., flavins, and extracellular enzyme activities without significantly decreasing cell viability. The findings suggested that membrane permeabilization is the dominant mechanism for the enhancement of extracellular bioactivity in S. oneidensis by DSSN+. We further demonstrated that the interaction between DSSN+ and S. oneidensis cells enhanced biofilm formation and stability without compromising the overall biofilm activity. Taken together, our results suggest that membrane spanning conjugated oligoelectrolytes, of which DSSN+ is one of many possible molecular structures, may be applied to enhance extracellular bioactivity in bacteria toward more efficient biofilm-based biocatalysis.